Pulsars, those spinning, superdense neutron stars that send powerful
"lighthouse beams" of radio waves and light flashing through the Universe,
have been "lying about their ages," leading astronomers, and possibly
particle physicists, to erroneous conclusions for the past 30 years,
according to researchers using the National Science Foundation's Very
Large Array (VLA) radio telescope.

Bryan Gaensler, at the Massachusetts Institute of Technology (MIT) and
Dale Frail, of the National Radio Astronomy Observatory (NRAO), in
Socorro, NM, studied a pulsar that was thought to be 16,000 years old,
but found instead that it is at least 40,000 years old and may
be as old as 170,000 years. The results of their research are being
published in the July 13 issue of the scientific journal Nature.

"This means that much of what we thought we understood about the physics
of pulsars and neutron stars may be wrong," said Gaensler. "Neutron
stars are the densest objects in the Universe and provide important
physical tests of our most basic understanding of matter. Much of this
theory is based on a belief that we could accurately estimate their
ages. Our research indicates that these objects may be 10 times older
than we thought, and this could force much re-evaluation."

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A Mosaic of VLA images of Pulsar B1757-24 and the radio supernova
remnant G5.4-1.2, at different scales.

Gaensler and Frail studied a pulsar 15,000 light-years away in the
constellation Sagittarius that has travelled outside the shell of debris
from the supernova explosion that created it. The pulsar and the shell,
known as a supernova remnant, together are dubbed "The Duck," because
of their unusual appearance. Stars much more massive than the Sun end
their normal lives in violent supernova explosions, leaving behind an
extremely dense neutron star. Some of these neutron stars produce the
beams of electromagnetic radiation that characterize pulsars.

For the pulsar, designated B1757-24, to have travelled from the center of
the supernova remnant to its present position in 16,000 years, it would
have to have moved at about 1,000 miles per second, a particularly high
speed compared to other pulsars. Gaensler and Frail compared a 1993
VLA image of the region to one they made last year to measure the pulsar's
change in position over a known time, and thus to calculate its speed.
They were surprised to find the pulsar moved at a maximum of about 350 miles
per second.

"This means the pulsar took much longer to reach its current position, and
so it is a much older object than we had believed," said Frail.

Columbia University astronomer David Helfand, who, with Robert Becker of
the University of California-Davis, first drew attention to the unusual nature
of "The Duck" in 1985, said he was "secretly delighted" with the new VLA
measurements. "I was skeptical of the high velocity" attributed to the object
earlier, he said. The new work, he said, "clearly cautions us that a present
snapshot of a system does not always give a full picture of its history."

For years, astronomers have estimated the age of a pulsar by measuring the
rotation period of its neutron star and the tiny amount by which that
rotation slows down over time. The neutron star's powerful magnetic field acts
as a giant dynamo, emitting electromagnetic radiation as the star rotates.
That loss of energy slows the star's rotation, according to the standard
theory used for nearly three decades. A calculation based on the neutron
star's rotation period and its rate of slowing produces what astronomers
call its "characteristic age," which has been presumed to be the true age.

That presumption now is called into question. With the large difference
between B1757-24's "characteristic age" and the age required by
the new VLA measurements, "this pulsar has been lying to us about its
age," said Frail. The discrepancy could require astronomers to re-examine
many of their previous conclusions about neutron stars and how they work.

For example, Helfand pointed out that there are cases in which astronomers
concluded that a pulsar and supernova remnant, while nearby, are not
related because the pulsar's "characteristic age" was much younger than the
age calculated for the supernova remnant. "We now ought to re-examine those
cases," Helfand said.

While the older age for B1757-24 poses problems for some astrophysical
theories, the pulsar's slower speed actually helps current theory in
one area, according to Helfand. Neutron stars are thought to get a
"kick" because the supernova explosion that creates them is not
symmetrical. "The high velocity for this object was difficult to explain
through that theory," Helfand said. "The lower speed is easier to explain."

If the "characteristic ages" are not, in fact, the true ages of the
neutron stars, the implications extend beyond astronomy to particle physics.
"Neutron stars, as the densest objects in the Universe, provide a unique
laboratory for physics. Physicists look at neutron stars as a way of
showing how matter acts under these extreme conditions. Part of what they
need to know in order to draw proper conclusions is the age of the neutron
star. If that changes, so do many of their theories," said Gaensler, whose
work is supported by a Hubble Fellowship awarded by the Space Telescope
Science Institute.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.

A VLA image of Pulsar B1757 and the radio supernova
remnant G5.4-1.2, collectively known as "The Duck."